1. Field of the Invention
The present invention relates to a method for producing a gallium arsenide epitaxial wafer which is suitable for the production of a Schottky barrier diode, a field effect transistor (FET), and the like.
2. Description of the Related Arts
A single crystal of gallium arsenide (hereinafter referred to as GaAs) exhibits a higher mobility of electrons than a silicon single crystal, and therefore, is widely used for such devices as FETs, Schottky barrier diodes and the like operated at a high frequency region, particularly UHF and SHF regions.
An epitaxial wafer is used, for those devices, in which, for example, two single crystalline GaAs layers having low and high carrier-concentrations, respectively, are formed on a single crystalline GaAs substrate. The thickness and carrier concentrations of the two single crystalline layers are strictly controlled, and further, the carrier concentrations are steeply changed at the boundary of the two single crystalline layers.
Heretofore, proposals have been made for the provision of such epitaxial wafers. Namely, the amount of impurities fed, which determine the carrier concentrations, and the proportion of the amounts of gallium and arsenic fed are controlled. According to one of the proposals disclosed in Japanese Unexamined Patent Publication No. 55-77131, the feeding of impurities is interrupted during growth of the low carrier-concentration layer and the ratio of the amounts fed of elements of the IIIb and Vb group of the Periodic Table is controlled to be greater than 1.
The low carrier concentration-layer involves problems in that the carrier concentration thereof may vary due to the influence of the impurity concentration of the metallic gallium and arsine, i.e., the starting materials of GaAs, various crystal defects generated during the epitaxial growth, and other factors. These problems are not satisfactorily solved even by the method disclosed in Japanese Unexamined Patent Publication No. 55-77131.
The variance of carrier concentration of the low carrier concentration layer must be controlled within a predetermined value, for example, 10%. When producing epitaxial wafers for Schottky barrier diodes, such a variance of 10% or less has been attained at a ratio of 50% or less based on the number of production runs.